Not so long ago, Half Moon Run used to rehearse in a grotty room in downtown Montreal. "It had paper-thin walls," recalls singer and guitarist Devon Portielje, "vomit on the carpets and a bunch of crazy metal bands playing non-stop, long-form assault music, for hours." Still, it proved a welcome escape from their other lives – dishwashing, music school, psychology degrees.

Two years on, their lives have changed immeasurably. This warm morning, the band are unfolding themselves from their van in Newcastle, looking only faintly blurred by their journey down from Glasgow. It has been a furious summer: a successful European tour, two much-lauded performances at Glastonbury, as well as turns at Green Man, Reading and Leeds, which had critics tipping them as "the new Mumfords". And amid it all, the release of a critically acclaimed album, Dark Eyes, a mixture of indie, pop and folk, strong on harmonies with delicate smatterings of guitar and a splash of electronica. The evolution of this band, however, is a little peculiar: they were never great friends, bonded by record collections and camaraderie.

Rather, their meeting was near-accidental. Dylan Phillips (drums, keyboard, vocals) and Conner Molander (guitar, keyboard, vocals) grew up on Vancouver Island, Canada. Their siblings were acquainted, and they were familiar enough with one another that, when they both relocated to Montreal, they began to play music together. They placed an advert on Craigslist, seeking a bassist or a drummer who might like Grizzly Bear, Fleet Foxes and Radiohead. It was answered by Portielje, neither a drummer nor a bassist, but a singer and guitarist who had moved from Ottawa to Montreal for a job promoting music, quit on his first day, and remained in the city perusing the small ads.

They sensed something special in their first jam – special enough to rearrange the band and install Portielje as lead vocalist. But for a long time, they barely knew how to talk to one another. "Socially, if there wasn't music involved, I probably wouldn't hang out with them at all," Phillips says. "At the beginning, I liked the mysterious aspect. It played a big part in the kind of music we were writing."

So they conversed through music, laboriously crafting, recording and rerecording the 12 songs that make up Dark Eyes. They played a few barely attended shows, signed with Indica, added a fourth member, Isaac Symonds, and toured Canada in support of Metric and Patrick Watson. It was an odd, hurtling time: they were developing not only their songs and a cohesive sound, but a bond that left them somewhere between friends and colleagues.

Today, the relationship between them has softened. They've become united by the strangeness of their new lives on the road. But they still remain in peculiar orbit of one another. In their van (on their next tour this autumn, they will be upgraded to a bus, to their great relief), they do not listen to music or make much conversation. "It's a quiet time," Portielje says. "I guess we share rooms and share beds, and the only time alone is when we're in the washroom." They fill the long, dreary hours on the road listening to their iPods and sport on the radio. "We talk a little about business," adds Molander, "but we're so together all the time, you have to have a bit of time alone. The van is as separate as we get from each other."

While some bands are close enough to use touring downtime to go sight-seeing, or at least eat and exercise together, Half Moon Run remain resolutely separate. "I don't think we have ever done a band excursion for pleasure," says Portielje, amused at the idea. "Any time we are in a spot where there might be a bed or internet access, it's sleep and emails."

It's a dynamic that still affects them after a tour's over. "We're really organised as a band," says Phillips, "So it's tough to do fun activities with friends. They want to go to the beach or something, and it's really unorganised – and my band brain is just going, 'Come on guys.' I'm thinking, 'There's an efficient way to plan this.'"

However, even off-tour, the requirements of day-to-day band business mean they now tend to see more of each other than anyone else, especially at this burgeoning stage of their career.

"Conner's having a party the day after we get back," says Portielje, "and we're all invited." They laugh.